Serial Attached SCSI is Coming to a Data Center Near You

Author: David Woolf, Senior Storage Research Engineer and Principle SAS Consortium Engineer,
University of New Hampshire-InterOperability Laboratory (UNH-IOL)

This is an exciting time for Serial Attached SCSI (SAS). Its development curve has been extraordinarily smooth and swift, products are now launching and market entry will accelerate as 2005 goes forward. The last of the dust clouds are dissipating, and it’s becoming clearer what role SAS will play in the near- and not-so-near-term data center market.

Greasing the Skids
The rapid SAS market ramp-up owes much to the forethought and careful planning of key engineering players and organizations like the SCSI Trade Association (STA). In fact, SAS’ smooth transition from the engineering white board to the marketplace should be taken as a model for successful technology development. This process succeeded for two key reasons: First, SAS didn’t reinvent the wheel. Its development incorporated and built upon older, well-established technologies such as SCSI and the trusted XAUI (see definition two paragraphs below) physical layer. Second, interoperability tests were built into the development process from the start. Amazingly, this is not necessarily the rule with new technologies.

Standards tend to run into interoperability hurdles when every aspect of the technology is new, from the physical layer all the way up the stack. They run aground rapidly when developers take on too much, attempting to solve multiple problems with enticingly new technologies and protocols. SAS has avoided this pitfall by confining itself to solving a single problem (shortcomings in SCSI) and using tested, field-proven technologies as a foundation.

SAS has a physical layer based on 10 Gb Ethernet’s Attachment Unit Interface specification (XAUI), defined by the Institute of Electrical and Electronics Engineers (IEEE). XAUI uses four serial channels running in parallel at 3.125 Gb/s to create a 10 Gb connection. SAS and Serial ATA (SATA) have taken this technology and used only one channel, running at 3.0 or 1.5 Gb/s to connect a disk and a host bus adapter (HBA). So the physical layer of SAS is not brand new. Similar flavors have already been developed and refined for XAUI, and are already on the market in the form of SAS’ complementary technology, SATA.

SAS and SATA share a similar out-of-band protocol (OOB) used to let end nodes identify each other as SAS or SATA devices and perform initialization. SAS uses OOB both for initialization and for interoperability with SATA devices. At the encoding layer, SAS uses 8b-10b encoding to create transmission characters and primitives from bits. This is the same encoding method used by Fibre Channel and Gigabit Ethernet. By using such a tried and true encoding method, SAS ensures that there won’t be any surprises at this layer when the technology is deployed.

The Outlook for the Data Center
But aside from simply replacing parallel SCSI, market watchers ask, why would anyone want to deploy SAS in the data center? For one thing, SAS will be able to support many more drives than traditional SCSI. SCSI was hampered by a 16-device limit on each bus. SAS expanders allow a single SAS HBA to connect to thousands of disk drives. This alone will lower the cost of deploying SAS, relative to SCSI, since fewer HBAs will be needed.

In addition to its capacity, SAS is designed to be extremely flexible, which makes it ideal for nearline storage. Online storage typically implies storage that needs to be available all the time, and is usually implemented with Fibre Channel or SCSI disks. Offline storage implies data that must be kept, but will rarely be accessed and is best kept on slower but reliable tape.

In contrast, nearline storage is gaining traction in the market as a new class of storage that requires fast but infrequent access. Typically, nearline storage must be faster than tape, but also inexpensive. Many nearline arrays are populated with high capacity, low cost, low reliability parallel ATA disk drives.

Accordingly, SAS enclosures are now being designed so that they can be populated with either SAS or SATA disk drives, due to their similar physical connectors and signaling conventions. Thus enclosures can be bought and used for either online or nearline storage, and all that needs to change are the disks populating the enclosure, since both SAS expanders and HBAs will be able to talk to SATA disk drives. That translates into a solid, cost-effective value proposition that many organizations—whether already married to Fibre Channel or not—will almost certainly embrace.

SAS is built on a solid foundation, offers remarkable flexibility and has strong support from the component manufacturers. And its price point is proving to be not too far off from parallel SCSI. Considering these advantages, SAS will almost certainly appeal to data managers as the best means available to contain infrastructure costs in the data center.

David Woolf is the principle SAS Consortium engineer at the University of New Hampshire InterOperability Laboratory (UNH-IOL). The UNH-IOL tests SAS and SATA products for interoperability and conformance through STA plugfests as well as its own detailed interoperability, physical layer and PHY layer conformance-testing programs.
More information about the UNH-IOL’s SAS testing services is available at

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.